This invention relates generally to the field of integrated optics chips or devices and more particularly to the field of multifunction integrated optics chips such as those having integrated optic components formed on lithium niobate (LiNbO3) substrates. Integrated optics components formed on such chips include waveguides that may be arranged to function such as optical couplers and phase modulators. Multiple functions may be incorporated on a single device, which eliminates losses and errors associated with interfacing separate devices.
Multifunction Integrated Optical Chips (MIOC""s) are usually fabricated in large numbers on three to four inch wafers of lithium niobate (LiNbO3) using conventional photomasks, vacuum deposition, chemical baths, proton exchange, diffusion and etching techniques to form large numbers of identical components at low cost and with high reliability. MIOC""s capable of performing many functions such as polarization splitting/combining and modulating are used in fabricating middle and high accuracy fiber optic gyroscopes (FOG""s) or rotation sensors. The FOG uses the Sagnac effect to measure rates of rotation about an axis perpendicular to a coil of optical fiber. MIOC""s may also be used in forming other fiber optic sensors such as hydrophones or geophones that rely on the principles of the Mach-Zehnder or Michelson interferometer.
A fiber optic gyroscope includes means for introducing counterpropagating waves from an optical signal source into an optical fiber coil. Rotation of the coil about an axis perpendicular to the plane of the coil produces a phase difference between the clockwise and counter-clockwise wave via the Sagnac effect. The phase shift occurs because waves that traverse the coil in the direction of the rotation have a longer transit time through the optical fiber coil than waves that traverse the coil in the opposite direction. The waves are combined after propagating through the coil. This combination of waves produces an interference pattern that may be processed to determine the rate of rotation. Techniques for determining the rotation rate are well-known in the art.
It is common practice to form a FOG to include a multifunction integrated optics chip (MIOC) between the optical signal source and the optical fiber coil, which is typically formed of polarization maintaining fiber. The MIOC typically includes a plurality of optical waveguides arranged to form a Y-junction. The base of the Y-junction is connected to the optical signal source while the arms of the Y-junction are interfaced with ends of the optical fiber coil. Optical signals input to the multifunction integrated optics chip divide at the Y-junction to form optical signals that are input to the ends of the optical fiber coil as the counterpropagating waves. After propagating through the coil, the waves enter the optical waveguides that form the arms of the Y-junction. The waves then combine in the Y-junction and are output from the base of the Y-junction to an optical fiber. The combined waves are guided to a photodetector that produces an electrical signal that is processed to determine the rotation rate.
The desired condition in a fiber optic rotation sensor is the transverse electric (TE) mode propagating in the optical fiber coil and in the optical waveguides without added path lengths. Propagation of transverse magnetic (TM) modes and TE modes having added path lengths are undesired conditions. Error sources such as polarization cross coupling, which adds a phase shift (or polarization non-reciprocity, PNR, which is associated with always having two polarization components possible in the fiber at all times), manifest themselves as additional optical path differences in direct competition with the Sagnac effect. These error sources cause phase bias and amplitude bias errors when they are modulated at the frequency used by the phase modulators in the MIOC. The bias component in the fiber optic rotation sensor due to polarization cross coupling is inversely proportional to the square root of the absolute value of the polarization extinction ratio. Extinction ratio is defined as ten times the logarithm of the ratio of the undesired power (the power of the undesired mode) to the desired power (the power of the desired mode) of the polarization modes expressed in decibels. Minimizing cross coupling (maximizing the absolute value of the extinction ratio) in the MIOC reduces this type of bias error.
As further background, integrated optics chips, such as those disclosed herein may be formed using processes and steps similar to some of those disclosed in U.S. Pat. No. 5,193,136, which issued to Chin L. Chang et al. on Mar. 9, 1993 for PROCESS FOR MAKING MULTIFUNCTION INTEGRATED OPTICS CHIPS HAVING HIGH ELECTRO-OPTIC COEFFICIENTS; U.S. Pat. No. 5,046,808, which issued to Chin L. Chang on Sep. 10, 1991 for INTEGRATED OPTICS CHIP AND METHOD OF CONNECTING OPTICAL FIBER THERETO; U.S. Pat. No. 5,393,371, which issued to Chin L. Chang et al. on Feb. 28, 1995 for INTEGRATED OPTICS CHIPS AND LASER ABLATION METHODS FOR ATTACHMENT OF OPTICAL FIBERS THERETO FOR LiNbO3 SUBSTRATES; U.S. Pat. No. 5,442,719, which issued to Chin L. Chang et al. on Aug. 15, 1995 for ELECTRO-OPTIC WAVEGUIDES AND PHASE MODULATORS AND METHODS FOR MAKING THEM; and U.S. Pat. No. 4,976,506, which issued to George A. Pavlath on Dec. 11, 1990 for METHODS FOR RUGGED ATTACHMENT OF FIBERS TO INTEGRATED OPTICS CHIPS AND PRODUCT THEREOF.
Each of the foregoing patents is assigned to Litton Systems, Inc. of Woodland Hills, Calif. Each of the foregoing patents cited above is incorporated herein by reference for the purpose of providing those skilled in the art with background information on how integrated optics chips or multifunction integrated optics circuits are made.
This invention is particularly directed to methods and apparatus for reducing polarization non-reciprocity errors in a MIOC as a result of both TM modes and TE modes that have traversed undesired optical paths and then coupled into an optical waveguide formed on the MIOC.
If the gyro bias is significantly reduced, there is the potential to reduce the fiber costs by replacing polarization maintaining fiber with less expensive single mode fiber, or using a shorter length of polarization maintaining fiber than is presently used. There is also the potential to support increased gyro accuracy.
The present invention is designed to extinguish or trap the various light paths that could potentially cross couple through reflections off the top, bottom and sides of an integrated optics chip.
The present invention addresses the problem of light scattered from more than one source including the pigtails, y-junction and other possible material or defect-induced sources along the waveguide. The new design also addresses the side reflected beam paths. This is of particular importance when the sides of the chip have an enhanced reflectivity due to either a polished surface or a coated surface leading to higher reflectivity over a ground or saw cut surface or due to a wide chip. The present invention results in a mechanically stronger product and is also more resilient to the handling due to normal processing. The present invention may also have an advantage with the completely sealed top surface preventing data errors at the gyro level due to data spikes from arcing as well as an improved yield due to handling during processing and installation.
An integrated optics chip according to the present invention comprises a substrate formed of an electrooptically active material and an optical waveguide network formed on a first surface of the substrate. The optical waveguide network has an input facet where an optical signal may be input to the optical waveguide network and an output facet where optical signals may be output from the optical waveguide network. The integrated optics chip also comprises a trench formed in the bottom surface and extending into the substrate toward the optical waveguide network to a depth of at least 70% of the thickness. The trench is arranged to prevent light rays incident thereon from inside the substrate from propagating to the output facet. In particular, the trench prevents light scattered at the input facet from reflecting from the bottom surface of the substrate to the output facet.
The integrated optics chip according to the present invention preferably further including a cover mounted to the top surface of the substrate. The cover preferably extends substantially the entire length of the substrate.
The integrated optics chip may include a plurality of trenches formed in the bottom surface and extending into the substrate toward the optical waveguide to depth of at least 70% of the thickness. The trenches may extend in the substrate to about 95% of the substrate thickness.
The integrated optics chip according to the present invention may further include one or more grooves formed in a side of the substrate and cover.
The integrated optics chip according to the present invention may further including a light absorbing material in the trench and in the grooves.
The integrated optics chip according to the present invention preferably further comprises an electrode pattern formed on the top surface of the substrate adjacent the optical waveguide network and a plurality of access electrodes formed on sides of the substrate and cover to provide electrical signals to the electrodes.
An appreciation of the objectives of the present invention and a more complete understanding of its structure and method of operation may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings.